1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
The coverage area of a wireless communication system is typically divided into a number of cells, which may be grouped into one or more networks. Access terminals located in each cell may access the wireless communications system by establishing a wireless communication link, often referred to as an air interface, with a base station associated with the cell. The access terminals may include devices such as mobile telephones, personal data assistants, smart phones, Global Positioning System devices, wireless network interface cards, desktop or laptop computers, and the like. As an access terminal moves between cells in the wireless communication system, the access terminal may periodically provide route update messages (also known as location update messages) that inform the wireless communication system of the access terminal's current location. Due to the mobility of the access terminal, the location of the cell where the most recent route update message was received is used to estimate the current location of the access terminal.
In some activity states, such as the idle or dormant mode or when the access terminal has been powered down, the access terminal may stop sending route update messages even though it may continue to move through the cells in the wireless communication system, until some condition is met (e.g., when the access terminal crosses the boundary of the sub-net associated with the last route update message, a new location update with the new sub-net is sent). Accordingly, the wireless communication system may not know which cell contains the access terminal when information becomes available for delivery to the access terminal. A wireless communication system may then attempt to reach the access terminal by sending paging messages over a plurality of cells belonging to a paging area determined by the network based on the information it has about the last known access terminal location, e.g., over the cells belonging to the last known sub-net. The paging messages contain information that indicates to the access terminal that information is available for transmission to the access terminal. If the access terminal receives the paging message, it may provide a paging response to a base station of a cell. The paging response typically indicates that the access terminal is available to receive the information and may also provide information indicating how to route the information to the access terminal.
Both the paging messages and the route update (or registration) messages represent system overhead. Accordingly, the wireless communication system is generally designed to meet two conflicting objectives: reducing the overhead from the paging load and reducing the number of route update messages transmitted by the access terminal. The paging load is typically minimized when the location of the access terminal is known with relatively high accuracy so that each paging message can be transmitted to a relatively smaller number of cells. However, increasing the accuracy of the location of the access terminal requires transmitting a larger number of route update messages during a given period. In contrast, reducing the number of route update messages transmitted by the access terminal may reduce the accuracy of the access terminal location estimation by the wireless communication system, which typically results in each paging message being transmitted to a relatively large number of cells.
The conventional solution to this problem is to define sub-nets that include the cells serviced by a plurality of base stations. The access terminals may then transmit route update messages when they cross from one sub-net to another sub-net and the wireless communication system may begin the paging process by providing paging messages via the base stations in the sub-net indicated by the most recently received route update message. For example, the geographic area served by the wireless communication system may be divided up into multiple sub-nets that encompass the cells serviced by groups of 50 base stations. Access terminals in the wireless communication system may then provide location updates when they cross a cell boundary between the groups of 50 base stations and the wireless communication system may provide paging messages via the groups of 50 base stations in the sub-nets.
However, the number of users and the geographical area served by wireless communication systems is increasing, which results in an increase in the size and number of cells in a typical sub-net. Since conventional route update messages are triggered by crossing a sub-net boundary, access terminals may travel through numerous cells without providing any route update messages to the wireless communication system. Consequently, the wireless communication system may be required to provide paging messages to a large (and likely increasing) number of cells in order to locate the access terminal. The wireless communication system may therefore have to devote a larger percentage of system resources to supporting overhead associated with providing paging messages to large numbers of cells in the sub-nets.
One technique for reducing the number of cells that are paged is called radius-based paging. Radius-based paging techniques force the access terminal to provide a route update message when it travels a distance larger than a predetermined radius from a center of the cell where the access terminal was last seen. Alternatively, the access terminal may provide a route update message when it travels a distance larger than a predetermined radius from a group of cells covered by a radio network controller associated with the access terminal. The wireless communication system may then page the access terminal by providing a paging message to the cell where the access terminal was last seen. If the access terminal does not respond to this paging message, the wireless communication system may page the cells in the area defined by the predetermined radius. Paging messages may be provided to cells in increasingly large areas (indicated by increasingly large radii) if the access terminal fails to reply to a previous paging message.
Although radius-based paging may be preferable to subnet-based paging when the subnets include a large number of cells, radius-based paging still has a number of drawbacks. The system overhead associated with providing paging messages to all of the cells within the predetermined radius may consume a large percentage of the resources of the wireless communication system. The number of cells within a predetermined radius may be reduced by decreasing the radius, but this will result in an increase in the frequency of transmitting route update messages by the access terminal. Consequently, the system overhead associated with providing the route update messages may increase and consume a larger percentage of the resources of the wireless communication system. Power consumption by the access terminal may also be increased when the number of route update messages increases. Furthermore, relatively long paging delays may be experienced as the wireless communication system attempts to locate the access terminal in cells at increasing distances from the cell where the access terminal was last seen.